Area Forecast Discussion
Issued by NWS La Crosse, WI

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733
FXUS63 KARX 201723
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1123 AM CST Wed Feb 20 2019

.SHORT TERM...(Today and tonight)
Issued at 327 AM CST Wed Feb 20 2019

Main short-term fcst concerns are snow amounts/possible FZDZ today
into this evening.

Data analysis at 06z had increasing moisture/forcing/lift under
diffluent mid/upper flow spreading across the region. This ahead of a
strong shortwave trough lifting NE across the central plains.
Widespread snow was developing/spreading NE across the region with
the leading edge already north of KAUM-KDEH-KPDC-KMRJ line, and
lifting NE around 15 mph. Most vsbys in the snow from SE SD to SW WI
and across much of IA in the 1/2 to 2 mile range, with reports of
1/2 to 1 1/2 inches of snow in a rather short period of time making
roads slippery.

Model runs of 20.00z initialized well. Models offer quite similar
solutions as the vigorous shortwave trough/energy lifts quickly thru
the region today then hgts rise behind it tonight as strong
troughing re-loads in the SW CONUS. Short-term fcst confidence is
good this cycle.

Deep layered forcing/lift, from lower level warm advection/
isentropic lift and some F-gen thru strong 500-300mb PV advection
and divergence aloft with at least partly coupled 300-250mb jet
maxes, to be lifting across the fcst area with the shortwave this
morning into early this afternoon. This with around 0.50 inch of PW
in the inflow airmass. Model/ensemble consensus of QPF production
and snow-water ratios of 15:1 show quick-hitter snow event for today
looks on track with totals of 4 to 8 inches, perhaps locally a bit
more west a KRST to KOLZ line. Snowfall rates of 1 inch/hr looking
likely at times for much of the area this morning. Have expanded
winter storm warning into Jackson/Clark/Taylor counties with 6+
inches of snow now expected there, with layout of the rest of the
headlines appearing good at this time. Snow on track to diminish
from SW to NE during the mid afternoon thru early evening hours
as the mid level shortwave passes and lift wanes.

Still potential for loss of ice nucleation in the cloud tops the
southeast side of the fcst area from mid morning thru this afternoon
as mid level drying would work around the passing shortwave.
Continued with a mention of some -FZDZ across roughly the SE 1/4 of
the fcst area much of today, with a light glaze of ice along with
the snow accumulations.

.LONG TERM...(Thursday through Tuesday)
Issued at 327 AM CST Wed Feb 20 2019

For Thursday thru Friday night: main fcst concerns this period are
returning precip chances/types with the next system Fri night.

Model runs of 20.00z in good agreement for rising hgts/broad ridging
aloft to build across the Upper Midwest Thu thru Fri as strong
shortwave troughing spins east along the US/MEX border. Models
offering a tightening consensus on this troughing as it moves into
west TX Fri night, but detail differences with the northern stream
portion of the trough into the northern plains Fri night. Even with
some differences Fri night, fcst confidence this period is generally
good.

Much of this period looking to the quiet-between-systems period as
high pressure from the plains builds east across the region under
the rising hgts/broad ridging/SW flow aloft. A bit of a cooler day
Thu with some weak cold advection over another 4 to 8 inches of
fresh, high albedo snow cover. Would have to watch Thu night for
lows bottoming out again Thu night (like Tue morning) with the
high overhead and the deep snow cover. However mid/high level
moisture streaming NE ahead of the next trough already progged
over the area Thu night. Looks like lows perhaps Thu evening, then
steady/slowly rising temps overnight. A milder day Fri as
stronger low level warm advection spreads north across the area,
even under mostly cloudy skies. Consensus highs near to a bit
above normal Fri reasonable.

Departing high Fri leaves a drier airmass over the area for Fri, but
increasing moisture ahead of both the southern and northern stream
portions of the trough to overcome this Fri night, with enough
moisture/lift in the column for precip chances Fri night. Precip
type is problematic with the warm advection from Fri pushing an
elevated warm layer northward into the area. There are also
potential issues with warmer cloud tops and lack of ice nucleation.
Perhaps some -SN across the fcst area Fri evening, then more of a
wintry mix overnight with a stronger signal for loss of ice in the
clouds.

For Saturday thru Tuesday (days 4 to 7): main fcst concern this
period in the strong system lifting NE across the region Sat into Sun
and precip types/amounts with it.

Medium range model runs of 20.00z offering a tightening consensus on
the next strong SW flow trough/shortwave lifts out of TX Sat to
lower MI by 12z Sun. Good agreement on quasi-zonal flow over the
region Mon then another shortwave trough to approach from the west
on Tue. Day 4-7 fcst confidence is average this cycle.

Attention this period is squarely focused on the SW flow wave to
lift toward/across the region Sat/Sat night. Present consensus track
puts the fcst area in the deformation band of precip on the NW side
of this system as it passes. Initially the column is looking rather
warm for Sat with BL temps in the 30s 850mb temps above 0C across
much of the fcst area. Bulk of any precip Sat looking to be -RA or a
-RA/-FZRA mix. By Sat night as the mid level low/shortwave approach
broad/strong dynamic lifting of the column cools it with precip Sat
night rapidly changing to snow. Several inches of snow then possible
Sat night into Sun morning, along with strong winds for blowing/
drifting of any new snow. High precip chances look in order for
later Sat into Sun morning, but given inherent trickiness with
precip types driven by BL temps and warm layers aloft, precip types
and any snow amounts are far from certain. And all this could still
shift north or south of the fcst area with later model runs. For now
the model/ensemble consensus in the fcst grids (its still 4-5 days
away) is a good place to start. After a brief break Sun night into
Mon, the next shortwave coming east across the northern plains
spreads snow chances (colder column with this one) back into the
fcst area for Tue.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1123 AM CST Wed Feb 20 2019

Snow will continue through mid afternoon with primarily IFR
visibilities before conditions gradually improve through late
afternoon as the snow ends. MVFR ceilings will persist into
tonight with clearing expected after midnight tonight and a
return to VFR conditions expected. SE winds will become SW/W late
this afternoon and tonight with occasional gusts to 20-25 kts at
times.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning until 6 PM CST this evening for WIZ032>034.

     Winter Weather Advisory until 6 PM CST this evening for
     WIZ041>044-053>055-061.

     Winter Storm Warning until 9 PM CST this evening for WIZ017-029.

MN...Winter Storm Warning until 6 PM CST this evening for MNZ079-
     086>088-094>096.

IA...Winter Storm Warning until 6 PM CST this evening for IAZ008>010-
     018-019-029.

     Winter Weather Advisory until 6 PM CST this evening for IAZ011-
     030.

&&

$$

SHORT TERM...RRS
LONG TERM...RRS
AVIATION...JM



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